As personal computers, workstations, and the like have become widespread in recent years, a large amount of research has been conducted into the magnetic tapes (known as back-up tapes) that are employed as external recording media for recording computer data. In the development of magnetic tapes for such uses, particularly as computers have decreased in size and increased in information processing capability, there has been a strong demand for increased recording capability to increase recording capacity and achieve size reduction. In the area of magnetic disks, as well, the rapid development of information technology is spawning a demand for the development of magnetic disks of ever greater density and capacity.
In the magnetic recording media developed thus far, a magnetic layer comprising a ferromagnetic hexagonal ferrite powder in the form of iron oxide, Co-modified iron oxide, or CrO2 dispersed in a binder that is coated on a nonmagnetic support has been widely employed. In these, the use of ferromagnetic metal powder and ferromagnetic hexagonal ferrite powder as magnetic powders is known to afford good high-density recording characteristics. For example, in the case of magnetic disks, 10 MB MF-2TD and 21 MB MF-2SD high-capacity disks employing ferromagnetic metal powder with good high-density recording characteristics are known. High-capacity disks employing ferromagnetic hexagonal ferrite powder in the form of 4 MB MF-2ED and 21 MB flopticals are also known. However, today, with sharp increases in the quantity of data being handled, even these magnetic disks do not afford adequate recording capacity and there is demand for magnetic disks of even greater capacity.
In the field of magnetic tapes, technologies of reducing the layer thickness of magnetic tapes to permit high-density recording are advancing. Numerous magnetic tapes having a magnetic layer thickness of equal to or less than 2 μm have appeared. With the high densification of magnetic recording media, a demand for greater coating smoothness has come, and the trend in magnetic material has been toward microparticles. However, when a magnetic layer of equal to or less than 2 μm in thickness is directly coated on a support, the surface of the magnetic layer is affected by additives in the magnetic layer such as abrasives and carbon, aggregates of magnetic powder, and the nonmagnetic support, rendering the magnetic layer rough and prone to exhibit deterioration in electromagnetic characteristics and dropout.
One means of solving this problem is to provide a nonmagnetic layer beneath the magnetic layer and thinly apply a highly concentrated magnetic coating liquid by a simultaneous multilayer coating method (Japanese Unexamined Patent Publication (KOKAI) Showa Nos. 63-191315 and 63-187418). The use of such simultaneous multilayer coating methods yields good electromagnetic characteristics even in a particulate magnetic recording medium having a thin magnetic layer.
However, the surface state of the nonmagnetic support in a magnetic recording medium having a thin magnetic layer greatly affects nonmagnetic layers and magnetic layers positioned over them. Thus, when employing a support of poor surface smoothness, the problem of reduced running durability occurs in addition to problems such as reduced electromagnetic characteristics and dropout. Further, in the magnetic recording media actually employed in digital VRC systems and DDS-4 systems, for example, high variation in the thermal expansion coefficient and moisture expansion coefficient during recording and reproduction causes variation in the thermal and moisture expansion coefficients of the magnetic recording medium itself, creating problems in the form of deterioration of electromagnetic characteristics and durability. Thus, further improvement is necessary.
To improve the support, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 6-234907 describes the specification of the size of the filler incorporated into the support, and Japanese Unexamined Patent Publication (KOKAI) Heisei No. 8-187774 and Japanese Unexamined Patent Publication (KOKAI) Nos. 2000-57558 and 2000-336186 describe the use of clay minerals as the fillers incorporated into the support. However, none of the techniques described in the above-cited art is capable of improving electromagnetic characteristics and durability in a particulate magnetic recording medium having a multilayer-configuration with a thin magnetic layer and a nonmagnetic layer.
Accordingly, it is an object of the present invention to provide a particulate magnetic recording medium for high-density recording having a low error rate and good running durability.
The present inventors conducted extensive research into the thermal expansion coefficient and moisture expansion coefficient both in the longitudinal and width directions, resulting in the discovery that incorporating a clay mineral coated with an organic material into a nonmagnetic support yielded a magnetic recording medium with little variation in thermal and moisture expansion coefficients that afforded good electromagnetic characteristics and durability; the present invention was devised on that basis.